1. Field of the Invention
The present invention relates to a process for manufacturing polyethylene and a plant for conducting said process.
2. Description of Related Art
Polyethylene, in particular low density polyethylene (LDPE) is in general manufactured in a high pressure process using a tubular reactor. The exothermic polymerization reaction is carried out under operating pressures between 500 and 4000 bar and temperatures between 165 to 340° C. The polymerization of ethylene is started by free radical initiator, usually using peroxides or oxygen.
These processes are highly integrated processes requiring complex plant networks. A LDPE production plant comprises usually one or multiple compressor units for compressing the ethylene feed, a preheater for pre-heating the ethylene feed and opt. other substance, a tubular reactor for the actual polymerization process of the ethylene feed coming from the preheater, a high pressure separator (HPS) for separating the polymer monomer mixture leaving the tubular reactor and a low pressure separator (LPS) for further separation of the polymer monomer mixture. The molten polymer is then passed from the LPS to a finishing section including an extruder. The monomer phase comprising ethylene as main component is usually recycled to the ethylene feed entering the compressor units (U.S. Pat. No. 6,596,241 B1, US 2005/0192414 A1).
Low density polyethylene is characterized by a relative high number of short side chains. The short chain branches regulate the flexibility and thus density of the polymer. An increased number of short chain branches improves the flexibility and optical properties, but reduces the mechanical strength. The melt strength in turn is influenced by the molecular weight tail and number of long chain branches.
In order to regulate the molecular weight so called chain transfer agents are added to the ethylene feed. These chain transfer agents promote the transfer of a growing polymer chain to another molecule thereby reducing the average molecular weight of the final polymer.
Besides chain transfer agents comonomers can be added to the ethylene feed in order to modulate the final polymer properties. It is for instance desirable to obtain polymers having a high degree of non-saturated bonds which are available for further chemical reactions, such as e.g. introducing functional groups or for simplifying cross-linking by the use of peroxides or irradiation. Polyunsaturated compounds having at least two non-conjugated double bonds of which at least one is terminal like 1,7-octadiene or 1,9-decadiene have to be proven to be excellent comonomers for this purpose (EP 0 647 244 B2).
The ethylene feed is combined with one or more comonomers and one or more chain transfer agents before entering the compressor unit or preheater. The polymerization initiator is later added to the reaction mixture, preferably before or after the mixture has entered the tubular reactor in order to start the polymerization reaction.
It turned out that when feeding a mixture of ethylene and comonomer, in particular octadiene, to the compressor unit and subsequently to the reactor the output of octadiene-polyethylene-copolymer is reduced compared to the synthesis rate of polyethylene-homopolymer i.e. polyethylene without a comonomer. Furthermore, feeding a mixture of ethylene and comonomer to the compressor unit required also subsequently an increased polymerization initiator e.g. peroxide feed in order to maintain the output. This effect increases even further with increasing octadiene concentration as comonomer.
It furthermore turned out that when feeding a mixture of ethylene, comonomer like octadiene, and/or chain transfer agent to a preheater unit the preheater exit temperature is lower than expected and thus has a negative effect on the overall efficiency of the whole production process. The temperature drop in the preheater is in particular caused by a prepolymerisation of the comonomer octadiene and ethylene within the preheater unit without any addition of an initiator. This prepolymerisation makes it difficult to heat up the feed to the temperatures required in the synthesis reactor for starting the polymerisation reaction.